This version of the site works on Bitcoin TESTNET. It means that bitcoins aren't real and you can safely experiment without fear of losing any money. Be sure you install a Bitcoin TESTNET wallet though: read up our manual on how to do that in the Guides/FAQ section.

This is how many Bitcoin transaction confirmations you will wait until the buyer (be it you or your counterparty) can safely proceed with sending the payment. Beware that you should manually adjust (perhaps increase) the payment window if you increase the number of requried confirmations. That way, the contract won't be prematurely automatically canceled.